When subjected to variation in temperature, light, wind, or humidity, plants alter their physiology and development. Environmental conditions influence commitment to developmental events such as seed germination and flowering, as well as adjustments of growth leading to morphological variation, such as stem elongation, lateral bud inhibition (apical dominance), and leaf size. This plasticity of plant growth and form enables one to learn about the mechanisms of plant development by studying signal transduction pathways that link environmental signals to developmental responses. This proposal describes experiments designed to elucidate molecular mechanisms of light signal transduction. Light influences virtually all aspects of plant development, through signalling system distinct from the photosynthetic apparatus. These systems respond to far-red, red, blue, or UV light through the action of distinct photoreceptors. Mutations in the PHYB gene, encoding the red/far-red photoreceptor phytochrome B, cause several developmental phenotypes including elongated hypocotyls, early flowering, and increased apical dominance. Characterization of more phyB mutations will provide information on structural requirements for phytochrome B function. Screens for mutations that suppress or enhance the long hypocotyl phenotype of a phyB mutant will identify genes whose products function downstream in phytochrome B signal transduction, and possibly, in other light signalling pathways. Genetic and physiological experiments will reveal the roles of genes identified by the mutations in control of development by light. Map-based cloning of the most interesting loci will permit molecular reconstruction of the pathways controlling plant developmental responses to the environment. Understanding these mechanisms may allow more rational manipulation of agronomically important traits such as flowering, seed, germination, and height in crop plants, contributing to improvement in human nutrition. These experiments will also lead to fuller insight into basic mechanisms of plant growth.